Patent GB-1341288-A (GB)
Title: Exposure calculator and filter device for darkroom colour photography
IPC: G03B27/73
Filed: 1971-04-19
Abstract: 1341288 Exposure time calculator and filter device for colour photography KMS INDUSTRIES Inc 19 April 1971[12 Feb 1970] 21615/71 Heading G2A An exposure time calculator for colour photographic emulsions comprises an opaque member having three series of windows therein, means providing the windows of said series respectively with red, green and blue filters, the densities of the filters in each series being graduated in a step by step manner, each of said windows having a portion which tapers acutely and terminates at a point, and translucent means disposed adjacent said series of windows adapted to provide exposure duration indicia correlated to the densities of said filters provided in said windows. The windows may have a plurality of taper portions e.g. two taper portions which are opposed with their points contiguous. The calculator may be used with a projector having a diaphragm adjustable to various f/stops and the densities of the filters provided in said windows may be correlated to the f/stops e.g. adjacent graduated densities may correspond to a variation of one third of an f/stop. The means providing each of said filters may comprise a coloured translucent element secured in laminar relation with a gray translucent element having graduated density. The opaque member and translucent elements may comprise exposed and developed photographic films. The calculator may be a rectangular frame 56 and a number of ribs 58 defining openings 60, 62, 64, 66 and 68. Blue, green and red translucent filter strips may be secured in openings 66, 64 and 62 respectively. A gray scale having a step-by-step graduated density along its length may be mounted beneath each of the filters. An opaque mask 74 may be secured to the bottom of the frame 56 so that it lies under the filter elements and gray scale. The mask 74 has three series of translucent windows 76, 78 and 80 which underlie respectively openings 62, 64 and 66, a column of translucent portions 81 defining numbers which represent exposure times in seconds which column of numbers underlies opening 68 in frame 56, and another translucent portion 82 which underlies opening 60 in the frame to provide a writing space. After exposure the paper is developed and the test print looks like: - The correct exposure time is obtained by finding the faintest image where contiguous points are still sharply defined and reading off the exposure time from column 81. For blue (B) this will be the image third from the bottom. The correct exposure for blue light will therefore be 30 seconds, for green light 16 seconds and for red light 6 seconds in this particular case. The calculator may be used in combination with a filter device which comprises a holder having a series of openings within which are mounted respectively a diffusion filter, a blue filter, a green filter and a red filter, and mounting means adapted to be mounted on the lens barrel of a projector with which the calculator and filter device are to be used, said mounting means movably supporting the holder so that the filters can be moved successively into alignment with the lens barrel axis. The filter wheel 28 has a ring 32 for attachment to the lens barrel 30 and the ring 32 preferably has three threaded openings for mounting screws 34. The ring 32 has a tab 38 to which a wheel 40 is pivotally secured at 42. The wheel 40 has a base element 44 and a cover element 46 and these elements are apertured to provide a series of openings 48 whose centres are arranged circumferentially around pivot 42. The opening marked 'open' has no filter therein. The cover element 46 carries five position indicators 50, each one positioned diametrically opposite its respective opening 48.